I created an image of my machine (using Clonezilla) so that I could put a new operating system onto my machine and still have everything backed up.
How can I tell if the clone was accurate and complete?
There's at least two scenarios you should consider for your verifying your backup image.
If your true focus is to restore successfully when the worst happens; then should be more concerned with ensuring you can restore completely from that image, because until you actually simulate that your drive did fail and you need to fully recover, your backup isn't trustworthy enough.
When you successfully restore from your backup, you can "smoke test" a variety of applications and files, perform file searches, open media, save changes, then and only then can you have confidence in the image you created.
Quoting some good backup blog posts:
But if it's just to snapshot some files and settings then as (per the other answers)
Typically, the cloning program will have an option to verify the image/backup when you create it.
Assuming that you're using the image taken as a way to go back and grab files that you may have missed in your standard backup, you can extract files from the archive. Following the steps to mount and extract files from the archive will, in essense, also verify that the archive is valid without corruption issues.
Some suggested methods for extracting from Clonezilla files can be found here.